MCS:

The L4.verified Proofs

This is the L4.verified git repository with formal specifications and proofs for the seL4 microkernel.

Most proofs in this repository are conducted in the interactive proof assistant Isabelle/HOL. For an introduction to Isabelle, see its official website and documentation.

Setup

This repository is meant to be used as part of a Google repo setup. Instead of cloning it directly, follow the instructions at the manifest git repo.

Dependencies

For software dependencies and Isabelle setup, see the setup.md file in the docs directory.

Contributing

Contributions to this repository are welcome. Please read CONTRIBUTING.md for details.

Overview

The repository is organised as follows.

• docs: documentation on conventions, style, etc.

• spec: a number of different formal specifications of seL4

  • abstract: the functional abstract specification of seL4

  • sep-abstract: an abstract specification for a reduced version of seL4 that is configured as a separation kernel

  • haskell: Haskell model of the seL4 kernel, kept in sync with the C code

  • machine: the machine interface of these two specifications

  • cspec: the entry point for automatically translating the seL4 C code into Isabelle

  • capDL: a specification of seL4 that abstracts from memory content and concrete execution behaviour, modelling the protection state of the system in terms of capabilities. This specification corresponds to the capability distribution language capDL that can be used to initialise user-level systems on top of seL4.

  • take-grant: a formalisation of the classical take-grant security model, applied to seL4, but not connected to the code of seL4.

  • There are additional specifications that are not tracked in this repository, but are generated from other files:

    • design: the design-level specification of seL4, generated from the Haskell model.
    • c: the C code of the seL4 kernel, preprocessed into a form that can be read into Isabelle. This is generated from the seL4 repository.

• proof: the seL4 proofs

  • invariant-abstract: invariants of the seL4 abstract specification
  • refine: refinement between abstract and design specifications
  • crefine: refinement between design specification and C semantics
  • access-control: integrity and authority confinement proofs
  • infoflow: confidentiality and intransitive non-interference proofs
  • asmrefine: Isabelle/HOL part of the seL4 binary verification
  • drefine: refinement between capDL and abstract specification
  • sep-capDL: a separation logic instance on capDL
  • capDL-api: separation logic specifications of selected seL4 APIs

• lib: generic proof libraries, proof methods and tools. Among these, further libraries for fixed-size machine words, a formalisation of state monads with nondeterminism and exceptions, a generic verification condition generator for monads, a recursive invariant prover for these (crunch), an abstract separation logic formalisation, a prototype of the Eisbach proof method language, a prototype levity refactoring tool, and others.

• tools: larger, self-contained proof tools

  • asmrefine: the generic Isabelle/HOL part of the binary verification tool
  • c-parser: a parser from C into the Simpl language in Isabelle/HOL. Includes a C memory model.
  • autocorres: an automated, proof-producing abstraction tool from C into higher-level Isabelle/HOL functions, based on the C parser above
  • haskell-translator: a basic python script for converting the Haskell prototype of seL4 into the executable design specification in Isabelle/HOL.

• misc: miscellaneous scripts and build tools

• camkes: an initial formalisation of the CAmkES component platform on seL4. Work in progress.

• sys-init: specification of a capDL-based, user-level system initialiser for seL4, with proof that the specification leads to correctly initialised systems.

Hardware requirements

Almost all proofs in this repository should work within 4GB of RAM. Proofs involving the C refinement, will usually need the 64bit mode of polyml and about 16GB of RAM.

The proofs distribute reasonably well over multiple cores, up to about 8 cores are useful.

jEdit

We provide a jEdit macro that is very useful when working with large theory files, goto-error, which moves the cursor to the first error in the file.

To install the macro, run the following commands in the directory verification/l4v/:

mkdir -p ~/.isabelle/jedit/macros
cp misc/jedit/macros/goto-error.bsh ~/.isabelle/jedit/macros/.

You can add keybindings for this macro in the usual way, by going to Utilities -> Global Options -> jEdit -> Shortcuts.

Additionally, our fork of Isabelle/jEdit has an updated indenter which is more proof-context aware than the 'original' indenter. Pressing ctrl+i while some apply-script text is selected should auto-indent the script while respecting subgoal depth and maintaining the relative indentation of multi-line apply statements.

Running the Proofs

If Isabelle is set up correctly, a full test for the proofs in this repository for seL4 on the ARM architecture can be run with the command

L4V_ARCH=ARM ./run_tests

from the directory l4v/.

Set the environment variable L4V_ARCH to one of ARM, ARM_HYP, X64, RISCV64, or AARCH64 to get the proofs for the respective architecture. ARM has the most complete set of proofs, the other architectures tend to support only a subset of the proof sessions defined for ARM.

Not all of the proof sessions can be built directly with the isabelle build command. The seL4 proofs depend on Isabelle specifications that are generated from the C source code and Haskell model. Therefore, it is recommended to always build using the run_tests command or the supplied Makefiles, which will ensure that these generated specs are up to date.

To do this, enter one level under the l4v/ directory and run make <session-name>. For example, to build the abstract specification, do

export L4V_ARCH=ARM
cd l4v/spec
make ASpec

See the HEAPS variable in the corresponding Makefile for available targets. The sessions that directly depend on generated sources are ASpec, ExecSpec, and CKernel. These, and all sessions that depend on them, need to be run using run_tests or make.

Proof sessions that do not depend on generated inputs can be built directly with

./isabelle/bin/isabelle build -d . -v -b <session name>

from the directory l4v/. For available sessions and their dependencies, see the corresponding ROOT files in this repository. There is roughly one session corresponding to each major directory in the repository.

For interactively exploring, say the invariant proof of the abstract specification on ARM, note that in proof/ROOT the parent session for AInvs is ASpec and therefore run:

export L4V_ARCH=ARM
./run_tests ASpec
./isabelle/bin/isabelle jedit -d . -R AInvs

or, if you prefer make:

export L4V_ARCH=ARM
cd spec; make ASpec
../isabelle/bin/isabelle jedit -d . -R AInvs

in l4v/ and open one of the files in proof/invariant-abstract.